my %msg_uid_hash = ();
...

sub daemon {
...

  $new_msg_cnt = 0;
  # check current uid list against our last one
  my @msg_uid_list = $pop->Uidl();
  foreach my $uid (@msg_uid_list) {  
    $new_msg_cnt++ if !$msg_uid_hash{$uid};
  }

  # replace the old uid list with the current one
  %msg_uid_hash = map {$_ => 1} @msg_uid_list;

...

}